With the help of the add-on Worldwide Clock, you may display world clocks in the toolbar popup of your browser.

Simply select as many clocks as you wish from the popup's drop-down box. Please keep in mind that there are presently 34 things to pick from. You may also specify the daylight-saving time (-1, 0, or +1) before adding any item. Items on the toolbar popup are draggable, so you may move them to the desired location; all changes are automatically stored to memory. By default, 9 things are added to the toolbar popup, but you may delete them at any moment by clicking on the cross-sign next to each item (remove button at the top-right corner).